As the techniques studied by the inventors of the present invention, for example, following techniques are conceivable in an inertial sensor.
By virtue of progresses in semiconductor processing techniques and micro machining techniques (so-called MEMS techniques), a MEMS inertial sensor element which has a detection circuit and detects physical quantities by the detection circuit has been widespread. For example, there is a system in which the gradient of a cellular phone is detected by an acceleration sensor which is a type of inertial sensors, thereby varying the direction of an output screen. Moreover, there is also a system in which the rotation angle of a vehicle body is detected in real time by an angular velocity sensor which is a type of inertial sensors, thereby controlling the running state of the vehicle body. Since sensor elements are generally used for a long period of time at a high temperature under the environment having many noises, durability is required for the sensors.
A MEMS inertial sensor processed/fabricated from a silicon wafer comprises: fixed electrodes; movable electrodes; an inertial body; and others. When physical force acts on the inertial body, the moved distance of the inertial body is detected by an electrostatic capacitance change between the fixed electrodes and the movable electrodes, thereby calculating the physical amount acted on the inertial body.
When the MEMS inertial sensor is to be formed by using a silicon wafer, the silicon wafer is processed by the deep etching technique of silicon. The deep etching technique is the processing technique in which chemical etching mainly using SF6 gas and chemical film formation mainly using CF4 gas are repeatedly carried out. The thickness of the silicon wafer forming the inertial body is 400 to 750 micrometers in many cases, and processing time of several tens of minutes to several hours is required for one silicon wafer when calculated from the etching rate of the deep etching of silicon. The long occupancy time of the deep etching is not preferred from the viewpoint of manufacturing cost. Therefore, in order to reduce the processing time by reducing the area to be etched, dummy pattern which does not directly contribute to the sensor performance is provided in the periphery of the patterns of the inertial body, the fixed electrodes, and the movable electrodes.
Also, if there are the regions having different aspect ratios of processing parts in the wafer plane when the inertial body is to be formed, the etching rate is varied. This is a phenomenon called the micro-loading effect, in which the etching rate becomes lower as the opening of the etched region becomes smaller. When a silicon wafer having a thickness of 400 to 750 micrometers is processed, the time taken until completion of the processing by the deep etching becomes longer in the part having finer patterns due to the micro-loading effect. The variation of the etching completion time due to the micro-loading effect is dependent also on the layout pattern. The etching rate of a fine pattern which takes the longest time is one third to one fourth of the etching rate of a large pattern for which etching is completed in the shortest time. Due to this variation of the etching completion time, the part of the processing patterns other than the fine patterns which take the longest time is exposed to a chemical substance serving as an etchant although etching thereof has been completed, and the processing excessively progresses. Thus, the difference is caused between the dimensions of the top part of the inertial body and the dimensions of the bottom part of the inertial body, and unpreferably the inertial body cannot be processed as it is designed. In order to control the processing dimension variation by suppressing the variation in the opening of the etched region, dummy pattern which does not directly contribute to the sensor performance is provided in the periphery of the inertial body pattern.
The dummy pattern provided in order to reduce the processing time or to control the variation in the processing dimensions is referred to as a “peripheral conductor”. Other than that, a substrate fixing and supporting the movable electrodes, the fixed electrodes, and the dummy pattern and a conductive part such as a package surrounding or covering these structures are also referred to as “peripheral conductors”. The peripheral conductors include single-crystal silicon and the single-crystal silicon above which an insulating film and a conductive film are formed. Further, when the inertial body and the peripheral conductors are made of single-crystal silicon, a natural oxide film having a thickness of about several nanometers is formed on the surface thereof after processing.
Note that techniques relating to such an inertial sensor include, for example, the techniques described in Japanese Patent Application Laid-Open Publication No. 11-173851.